We study the largely unexplored transition between coherent and noise-seeded incoherent continuum generation in all-normal dispersion (ANDi) fibers and show that highly coherent supercontinua with spectral bandwidths of one octave can be generated with long pump pulses of up to 1.5 ps duration, corresponding to soliton orders of up to N = 600. In terms of N, this corresponds to an approximately 50 times increase of the coherent regime compared to anomalous dispersion pumping. In the transition region between coherent and incoherent spectral broadening we observe the manifestation of nonlinear phenomena that we term incoherent cloud formation and incoherent optical wave breaking, which lead to a gradual or instantaneous coherence collapse of SC spectral components, respectively. The role played by stimulated Raman scattering and parametric four-wave mixing during SC generation in ANDi fibers is shown to be more extensive than previously recognized: their nonlinear coupling contributes to the suppression of incoherent dynamics at short pump pulse durations, while it is responsible for non-phasematched parametric amplification of noise observed in the long pulse regime. We further discuss the dependence of SC coherence on fiber design, and present basic experimental verifications for our findings using single-shot detection of SC spectra generated by picosecond pulses. This work outlines both the further potential as well as the limitations of broadband coherent light source development for applications such as metrology, nonlinear imaging, and ultrafast photonics, amongst others.

We present the first demonstration of mid-IR supercontinuum generation directly pumped with picosecond pulses from a Thulium fiber-amplified gain-switched laser diode at 2 µm. We achieve more than two octaves of bandwidth from 750 - 4000 nm in step-index ZBLAN fiber with Watt-level average power and spectral flatness of less than 1.5 dB over a 1300 nm range in the mid-IR from 2450 - 3750 nm. The system offers high stability, power-scaling capability to the 10 W regime, and demonstrates an attractive route towards relatively inexpensive, versatile and practical sources of high power broadband mid-IR radiation.

Abstract-We report supercontinuum generation using a mode-locked VECSEL emitting 400-fs pulses at a 3-GHz repetition rate, amplified with a cascaded ytterbium-doped fiber amplifier system up to 40 W of average power. The pulses were then recompressed to their original duration via a high throughput transmission grating compressor, and used to generate supercontinuum in two samples of photonic crystal fiber (PCF); an all-normal dispersion PCF, and a PCF with a zero dispersion wavelength of 1040 nm, creating 20 dB spectral bandwidths of 200 nm and 280 nm respectively.

We report the importance of cross-phase modulation (XPM) on the coherence of a low-energy probe pulse co-propagating with a high-energy pump pulse that generates incoherent supercontinuum in all-normal dispersion (ANDi) fiber due to Raman amplification of quantum noise. By investigating numerous fiber and pulse parameters, we show consistently that for weak probe pulses, the XPM from the pump is the dominant influence on the degradation of the probe coherence. We show that the faster decoherence at the pump leading edge means that the probe coherence is reduced more significantly when the probe has a higher group velocity, i.e., when an orthogonally polarized probe is aligned to the fast (lower refractive index) axis of the fiber or when a co-polarized probe has a longer central wavelength. Simulations show that this effect occurs for both polarization-maintaining (PM) and non-PM ANDi fibers and can result in a probe decoherence rate that is higher than that of the pump. These previously unreported results extend our earlier scalar simulations showing incoherent supercontinuum within a single pulse.

A stable and self-starting femtosecond breathing-pulse Yb-fiber oscillator is reported, modelocked using the nonlinear polarisation evolution mechanism. A bifurcation between two distinct modes of operation is demonstrated experimentally, producing pulses with a single central wavelength in one state, or following adjustment of the intra-cavity waveplates, the emission of pulses with three distinct central wavelengths. The maximum bandwidth was 72 nm at the -10 dB level and the pulses were compressible externally to 70 fs with energies of 0.75 nJ. The multi-wavelength pulses reported here are significantly shorter than the pico-second pulses previously observed from similar modelocked multi-wavelength sources. Vector simulations based on the nonlinear Schrödinger equation show that the multi-wavelength behaviour is produced by overdriving the nonlinear polarisation evolution based saturable absorber at the peak of the pulse, leading to transmission of the two wings of the strongly chirped pulse. This new insight shows clearly that the three pulses output in the multi-wavelenght state are coherent. The agreement between simulation and experimental data shows nonlinear polarisation evolution based modelocked fiber lasers are a suitable platform for studying the nonlinear dynamics underlying the bifurcation of the output.